1. Field of the Invention
The present invention relates to capped polyoxyalkylene block polyethers. More particularly, the invention relates to an improved process for preparing such capped polyoxyalkylene block polyethers in high yield.
2. Description of the Related Art
Polyoxyalkylene block polyethers are well known commercial products having many uses, the most important of which is their use as nonionic surfactants. Polyoxyalkylene block polyether surfactants generally have both hydrophobic and hydrophilic blocks, and are described, for example, by Lundsted in U.S. Pat. No. 2,674,619 and by Jackson and Lundsted in U.S. Pat. Nos. 2,677,700 and 3,036,118. These references also disclose the preparation of such polyoxyalkylene block polyethers by oxypropylating an initiator molecule possessing two or more active hydrogens in the presence of a basic catalyst such as sodium or potassium hydroxide. The polyoxypropylene hydrophobe is then oxyethylated to produce external hydrophiles, or, in certain cases, the oxypropylation and oxyethylation may be reversed to produce "reverse" non-ionic surfactants having an internal hydrophile and external hydrophobes.
Although such polyoxyalkylene block polyethers have proven to be useful n many fields, certain applications have required properties not available without modifying their basic structure. One such modification which has proven useful is capping, or "end-capping."
Normal polyoxyalkylene block polyethers are hydroxyl terminated. Frequently, the nature of hydroxyl functionality is such as to impart undesirable properties in specific applications. When the polyoxyalkylene block polyether is difunctional, one of the hydroxyl functionalities may be eliminated by the expedient of beginning with a monofunctional initiator and appropriately altering the sequence of oxyalkylation.
For example, a tri-block polyoxyalkylene polyether may be conventionally prepared, as shown in the reaction scheme below, by first oxypropylating a difunctional initiator molecule followed by oxyethylation. In these reaction schemes, --OP-- and --PO-- represent oxypropyl residues derived from propylene oxide while --OE-- and --EO-- represent analogously derived oxyethyl groups. ##STR1## An analogous monofunctional, mono-capped tri-block polymer may be prepared by starting with a monol, R--OH, such as methanol, butanol, fatty alcohols, alkylphenols, or benzyl-alcohol and altering the oxyalkylation sequence as follows: ##STR2## Such mono-capped block polyethers where the cap is joined to the block polyether by an ether linkage are hydrolytically stable and have been shown to possess substantially different physical and chemical properties as compared to their non-capped analogues, including modified surface activity and increased thermal stability.
Unfortunately, this method cannot be used without further modification to prepare di-capped block polyethers. In the past, similar di-capped block polyethers could be prepared by first forming a mono-capped polyether and then joining two such polyethers utilizing a difunctional "linking" or "bridging" reagent. For example, in the process below, a diisocyanate is used as the "linking" reagent. ##STR3## The nature of R' in the diisocyanate may be aliphatic or aromatic. Unfortunately, such compounds contain two urethane linkages which are less stable, both hydrolytically and thermally, than a purely ether linked molecule.
An alternative to such a process is the preparation of a mono- or difunctional molecule followed by monocapping or dicapping, respectively, utilizing traditional capping reagents. Such capping is traditionally performed, for example, as in U.S. Pat. No. 3,393,242, by means of reaction of the hydroxyl functional polyether with an alkali metal or alkali metal alkoxide followed by reaction with an alkyl halide. For example: ##STR4## A methyl capped polyether is the result. Other capping agents such as dialkylsulfates may also be used in this process in place of the alkyl halide.
Unfortunately, the process of capping polyethers just described is subject to the disadvantage that the capping efficiency seldom, if ever, approaches 100 percent of theoretical. Even when an excess of the relatively expensive capping reagents are utilized, the capping efficiency does not generally exceed 90 percent of theoretical and is frequently far less.
It has now been surprisingly discovered that exceptionally high capping efficiencies may be obtained through the use of conventional capping procedures so long as cesium hydroxide is utilized as the polyoxyalkylation catalyst rather than conventional potassium or sodium hydroxide catalysts. The use of cesium hydroxide as a catalyst has not been previously suggested for the preparation of block polyether polyols.
The use of cesium hydroxide as a polyoxypropylation catalyst has been proposed in U.S. Pat. No. 3,393,243. According to this reference, the use of cesium hydroxide as opposed to conventional sodium or potassium hydroxide catalysts in the synthesis of polyoxypropylene glycols prevents the elimination reaction at the polyether chain terminus which ordinarily results in forming allylic unsaturation and, at the same time, lowers and broadens the molecular weight of the product polyoxypropylene glycols.
A mechanism for the elimination disclosed in U.S. Pat. No. 3,393,243 is discussed in Ceresa, Block and Graft Copolymerization, vol. 2, published by Wiley- Interscience, at page 18. The mechanism apparently involves hydrogen abstraction via a cyclic transition state and may be represented as follows: ##STR5## The unsaturation formed increases as a direct function of equivalent weight. Eventually conditions are established wherein further propylene oxide addition fails to increase the molecular weight.
When oxyethylation rather than oxypropylation is performed, as in the preparation of block polyethers, the use of cesium hydroxide as a catalyst has not been contemplated. The reason for this is that while it is readily conceived that polyoxypropylene glycols may undergo elimination by the above mechanism, the same cannot be true for polyoxyethylene glycols or for oxyethylated polyoxypropylene glycols containing more than one oxyethyl group. Thus, until now, such block polyethers have been prepared with less expensive sodium and potassium hydroxide catalysts.
For example, when a single oxyethyl group is added to a polyoxypropylene glycol, the elimination mechanism may be written thusly: ##STR6## However, when more than one oxyethyl group is present, this transition state cannot be achieved: ##STR7## Consequently, no elimination, no unsaturation formation, and therefore no lowering of molecular weight is expected during ethylene oxide addition, and, in fact, none has been detected heretofore.